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The Central Uplift of Spider Crater, Western Australia Large Meteorite Impacts and Planetary Evolution V (2013) 3025.pdf THE CENTRAL UPLIFT OF SPIDER CRATER, WESTERN AUSTRALIA. T. Kenkmann1 and M. H. Poelchau1, 1Institute of Earth and Environmental Sciences, Albert-Ludwigs Universität Freiburg, Albertstraße 23-B, 79104 Freiburg, Germany, [email protected]. Introduction: The Spider impact crater, Australia (13 Macrostructures and kinematic model: The central x 11 km Ø, N16°44’27’’, E126°05’21’’) named after uplift consists of two imbricate thrust stacks that sur- the peculiar appearance of its central uplift, is situated round the core of the structure, indicating N-S shorten- on the Kimberley-Plateau in Western Australia. J. E. ing with a preferred material transport top to southerly Harms was the first who recognized the structure direction (Fig. 1). Dipping of beds within the imbricate around 1960, but twenty years later a cryptovolcanic stacks ranges between 20-80° (40° mean dip) and origin was still favored [1]. Shoemaker and Shoemaker shows no systematic change from N to S. The traces of [2, 3] mapped the site in 1984-85. McHone [4, 5] and each thrust are bent and their strike gradually shifts Abels [6, 7] investigated the structure by means of from N to S. To explain the change in orientation of remote sensing analyses. While most authors favored the thrust slices from up range to downrange a very an oblique impact from N to NW to explain the nor- simple approach of vector summation in a horizontal therly dipping thrust stacks [2, 3], Abels [6, 7] argued plane might be helpful: An idealized flow field during that pre-impact topography and the (very) gently dip- crater collapse at a vertical incidence is represented by ping limbs of the broad Mt. Barnett syncline might purely radial convergent flow lines. This flow field is account for the asymmetric structure. combined with a flow field consisting of parallel flow We undertook a field survey of the central uplift at lines of outward decreasing magnitude that are point- Spider in 2007. The nearest infrastructure is the Gibb ing from up range to downrange. The latter trajectory River Road and the Mount Barnett Roadhouse. Access field is expected when a tangentially impacting projec- to the structure is restricted as it is part of the Mount tile transfers momentum to a target. Simple vector ad- Barnett pastoral lease under declared Aboriginal dition of the two flow fields results in curved trajecto- homeland. We present micro- and macrostructural ries whose orientations are perpendicular to the strike field data along with a simple kinematic model that of the thrusts. The imbricate stacks initiated up-range explains the structure of the central uplift. and migrated down-range simultaneously with the up- Microstructures: Twelve samples were analyzed for lift of the central core. Three-dimensional numerical petrography and shock metamorphism using optical modeling is necessary to fully resolve the trajectory microscopy, four from the central dome, six from the field in time and space. This is currently planned. thrust systems and two from outside the crater (Pente- cost sandstone). The samples of the core region are strongly deformed to monomictly brecciated Warton Sandstone (1.7 Ga), the oldest unit at Spider. This pure quartzitic sandstone shows some planar deformation features (PDFs) and very abundant planar fractures (PFs), many of them contain feather features (FFs) [8]. Shock effects propagate from quartz grains into the surrounding quartz cements suggesting that the rocks were completely lithified and of low porosity when the impact occurred. Shatter cones are ubiquitously pre- sent in the central dome. Pentecost sandstone samples from the surrounding thrusts show a quartz arenite with clayey and Fe-rich seams surrounding the quartz grains. The deformation varies from weakly deformed References: [1] Harms J. E. et al. (1980). Nature 286, to shocked. The shocked samples show abundant 704-706. [2] Shoemaker E. M. & Shoemaker C. S. (1988). transgranular fractures, PFs, FFs, and a few PDFs. GSA, Abstracts 20, A147. [3] Shoemaker E. M. & Shoemak- Shatter cones occur only in the vicinity of the central er C. S. (1996). AGSO Journal of Australian Geology & dome. Between the thrusts occur narrow zones of Geophysics, 16, 379-398. [4] McHone J. F. et al. (2002a) monomict and polymict lithic breccias. They contain a Met. Planet. Sci., 37, 407-420. [5] McHone J. F. (2002b) fine-grained lithic matrix (5-10 µm) in which angular LPSC XXXIII, abstract 1990. [6] Abels, A. (2001) LPSC fragments of quartz, chert, siltstone (50-100 µm) are XXXII, abstract 1408. [7] Abels, A. (2005) Aust. J. Earth Sci. embedded. 52, 653–664. [8] Poelchau, M.H. & Kenkmann, T. (2011) JGR 116. .
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